1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for effectively purifying carbon monoxides (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in exhaust gas from an automobile engine.
2. Description of Background Art
Exhaust gas purifying catalysts of three-way catalysts capable of simultaneously purifying carbon monoxides (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gas use noble metals, such as Pt, Rh, and Pd as active material. In recent years, in the exhaust gas purifying catalyst, cerium oxide (CeO2) has been receiving great interest in its capacity (oxygen storage capacity) of occluding or releasing oxygen in an atmosphere, and many attempts have been made to make the cerium oxide be contained in the three-way catalyst to adjust vapor atmosphere in oxidation reaction of CO and HC and in reduction reaction of NOx, to thereby produce improved purification efficiency. For example, various automobile exhaust gas purifying catalysts comprising the cerium oxide supported on alumina and the like, together with the noble metal, have been proposed.
However, when the cerium oxide is made to coexist with noble metal, dispersiveness of the noble metal is reduced and its low-temperature property (purifying property at starting under low temperature) is reduced. For avoidance of this problem, Japanese Laid-open (unexamined) Patent Publication No. Sho 62-71543 proposed that a coating layer of cerium oxide containing alumina is formed on the honeycomb catalyst carrier and a catalyst layer of noble metal is further formed on the coating layer, to produce the exhaust gas purifying catalyst of excellent in low-temperature property.
However, the exhaust gas purifying catalyst described by Japanese Laid-open (Unexamined) Patent Publication No. Sho 62-71543 is poor in heat-resisting property and hardly satisfactory in improvement of low-temperature property.
It is the object of the invention to provide an exhaust gas purifying catalyst that can develop good catalytic activity even in high-temperature endurance and also produce excellent low-temperature property.
The present invention provides a novel exhaust gas purifying catalyst comprising a coating layer supported on catalyst carrier, the coating layer comprising an outside layer formed on a front side thereof and an inside layer formed on an inner side of the outside layer, the outside layer comprising a noble metal previously supported on a heat-resisting oxide and a cerium complex oxide, the inside layer comprising a heat-resisting oxide on which no noble metal is supported.
This exhaust gas purifying catalyst of the present invention provides excellent low-temperature activity and also develops good catalytic activity even in high-temperature endurance. Accordingly, it can suitably be used for the automobile exhaust gas purifying catalyst.
It is preferable that the heat-resisting oxide supporting the noble metal thereon in the outside layer comprises at least one material selected from the group consisting of cerium complex oxide, zirconium complex oxide and alumina, and the heat-resisting oxide of the inside layer comprises at least one material selected from the group consisting of cerium complex oxide, zirconium complex oxide and alumina.
It is preferable that the cerium complex oxide comprises cerium and zirconium in the proportion of a cerium content being more than a zirconium content, and the zirconium complex oxide comprises zirconium and cerium in the proportion of a zirconium content being more than a cerium content.
Also, it is preferable that the cerium complex oxide is the heat-resisting oxide expressed by the general formula (1) given below:
Ce1-(x+Y)ZrxMyO2-zxe2x80x83xe2x80x83(1)
(In the formula, M represents alkali earth metal or rare earth metal, z represents oxygen vacancy, x represents a proportion of atom of 0.20-0.70, y represents a proportion of atom of 0-0.20, and 1xe2x88x92(x+y) represents a proportion of atom of 0.10-0.80), and the zirconium complex oxide is the heat-resisting oxide expressed by the general formula (2) given below:
Zr1-(a+b)CeaNbO2-cxe2x80x83xe2x80x83(2)
(In the formula, N represents alkali earth metal or rare earth metal, c represents oxygen vacancy, a represents a proportion of atom of 0.10-0.35, b represents a proportion of atom of 0-0.20, and 1xe2x88x92(a+b) represents a proportion of atom of 0.45-0.90).
It is preferable that the alkali earth metal represented by M in the general formula (1) and the alkali earth metal represented by N in the general formula (2) comprises at least one material selected from the group consisting of Mg, Ca, Sr and Ba, and the rare earth metal represented by M in the general formula (1) and the rare earth metal represented by N in the general formula (2) comprises at least one material selected from the group consisting of Y, Sc, La, Pr and Nd.
Preferably, the noble metal comprises at least one material selected from the group consisting of Pt, Rh and Pd. Also, it is preferable that the outside layer and/or the inside layer further comprises hydrosulfate, carbonate, nitrate and/or acetate of Ba, Ca, Sc, Mg and La.
It is preferable that both of the outside layer and the inside layer comprise alumina.
Further, it is preferable that a catalyst layer comprising a noble metal is further formed on the coating layer at an exhaust gas inflow side of the catalyst carrier.
The constitution that the catalyst layer comprising a noble metal is further formed on the coating layer at an exhaust gas inflow side of the catalyst carrier can allow the carbon monoxides (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gas to be purified in the vicinity of the exhaust gas inlet, so that the entire catalyst can be prevented from being deteriorated to provide the purification with efficiency.
Further, it is preferable that the noble metal of the catalyst layer comprises at least one material selected from the group consisting of Pt, Rh and Pd.